Method of treating metallic objects and resulting products



9 a "UN T D- TATE rAraNr QFFICE. 7

omams r, iransim, or new YORK, n. Y. QFNEW YORK, n. Y., 400

, B STGNOB TO JIADSENELL CORPORATION, I

BPOBATION OF NEW .YOBK.

METHOD OF TB EATI NG METALLIC OBJECTS AND RESULTING PRODUCTS.

Ho Drawing. Application fllcd m 21, -iia2o, 'sma1 No. 398,888. Renewed February. 10, mi.

To all whom mag concern: I f Be it known that I, CHARLES P. MADSEN,

. a citizen of the United States, and a resident of New York city, in the county and State of New York, have invented an Improvement in Methods of Treating Metallic Objects and Resulting Products, of which the following is a specification,

' 1 My invention relates to methods of treat ings of various metals in various thicknesses,- I b means of various coating processes, and to lie-products resulting from the apphcationof ing metallic oh'ects to enable thesame'to receive closely a herent and impervlous-ooatsuch methods. My invention finds particularly. successful application in" the treatment of metallic sheets, usually of iron or steel, preferablilof steel, to enable the same to aftei'war sgi eceive adhesive and impervious i'netallic coatings of such character as to enable theicoa-ted sheets to be afterwards shaped, although my invention isalso applicable to shaped or partially shaped articles.

For purposes of illustration I shall de- 4 I scribe, in the accompanying specification,

one form of my invention as more particularly a plied to the treatment of iron or steel ob ects, preferably objects of steel, and more a'rticularly in the form of steel sheets, inor er-to enable such steel sheets to receive adherent, impervious coatings of metal.

"Vhile various means of apply g the tallic. coating may be employed, such as melting, spraying, or.electrodeposition, I shall, in connection-With the-illustrative embodiment of my invention herein set forth, de-

- scribe the same as 211) lied in connection with the coating of meta s by electrodeposition. Moreover, while coating metals of varying composition may be-em loyed, such as nickel,

tin, lead or zinc, I sha l in the accompanying specification, by way of example, describe the use of zinc as the coating metal, 1t belng understood; however, that my 1nvention is not limited to the illustrative embodiments thereof which are herein set forth for purposes'of illustration only.

, live embodiments of m clearly understood that I do not wish in any various metals as applied by the ordinary methodstov iron or steel which has been treated by the usual cleaning processes, as by the usual chemical or electrochemical pickles. has generally admitted, however,

that' no close. adherence amounting to an alloy union is obtained between the steel or equivalent base and the coating metal, except possibly in the case of tin, applied hot. This. difference of opinion probably arises fromv the fact that previous investigators have not taken into suflicient account the effect of the thickness of the coating upon its degree of adherence. I have discovered,

among other things, that coatings less than a certain critical thickness, which varies for the various metals, and which in the case of nickel, applied by electrodeposition, for example, is between about two ten-thousandths an three ten-thousandths of an inch, when applied to steel or iron, treated by the ordinary cleaning methods, adhere much more closely than coatings which are greater in thickness than the critical thickness specified in the case of nickel, although such coatings do not constitute true alloy unions. By

critical thickness I mean such thickness for the electrodeposited coating that if such thickness is exceeded, the coatmg is impervious but non-adherent, while if such thicknessis equalled or if the. thickness of the electrodeposited coating is less than such critical thickness, the coating is adherent but is pervious. I

It is, however, clearly understood that deposits of a metal of a thicknessbelow about two ten thousandths to about three tenthousandths of an inch are all porous, coatings of these thicknesses being the ones which I have discovered possess the slight degree of adherence already referred to.

That is, taking-the case of the ordinary elec.

trodeposited coatings, for example, such coatings on steel treated by the ordinary cleaning or picklinglmethods, are porous up to the critical thickness, which is about three ten-thousandtlis of an inch, in the case of bases.

a Sufficient thickness to make the coatings relatively impervious, I find that .such' coatings lose the slight adherence which coatings of less thickness than the critical thickness referred to generally possess. The thicker coatings, as coatings from about five to about ten ten-tho'usandths of an inch or greater in thickness in the case of nickel, for example,'while less pervious, or even entirely impervious, have little or no adherence to the steel or other bases to which they. may be applied, and are readily separable from such bases.

No satisfactory explanations of rious and w ile I do not wish to be limited to any particular theory of the cause involved in and the principles underlying these phenomena, it is my present opinion that coatthese vaings below the critical thickness for any metal, as between about two ten-thousandths and about three ten-thousandths-of an inch in the case of .nickel for example, applied by electrodeposition, to take a particular example of one of the numerous methods of coating one metal with another, follow the surface crystal structure of the steel or equivalent bases to which they are applied, and tend to adhere to the separate crystals of the surface of the bases, with a consequent tendency to leave spaces or rifts at the boundaries of the crystals on the surface of the But where the thickness of the coating is increased, as to about five ten-thousandths to about ten ten-thousandths of an inch or more in thickness, as in the above example of nickel applied to the base by electrodeposition, the coating, the exact character of which otherwise depends on the factors of deposition, tends tOtlSSIllIlG its own physical form, independently of the form or character of the surface to which it is fied that the deposit applied thereto, as in case of nickel or zinc applied by electrodeposition, does not tend, even inthicknesses less than the critical thickness defined above, to follow the surface crystal structure of the base, but tends rather, from the very start,

to assume an independent, homogeneous and impervious structure, andto render the coating applied to a base treated by the method subsequently set forth not only impervious henomena have hitherto been offered,

mer.

downto the very smallest thicknesses, but also'remarkably adherent, even in thicknesses greatl exceeding; the critical thickness as de ned above. So strong is this adherence that it has led me to believe that the junction between the base and the coating is probably an alloy one. I have further discovered that by combining the method for treating the iron, steel or equivalent base as more fully set forth hereinafter, with the method of coating with zinc, particularly by electrodeposition, more fully set forth 4 hereinafter, I may obtain a zinc coating which is not only impervious for thicknesses down to one-half of one ten-thousandth of an inch, and adherent for all thick-- nesses, even for thicknesses much in excess of the critical thickness of zinc, which is from about two to about three ten-thourating from its base, or breaking during the working or shaping operation.

Referring to the illustrative embodiments of my invention mentioned above, which are herein set forth for purposes. of illustration only, I may use for the object to be coated a sheet of steel or iron, preferably the for- I now treat the iron or steel sheet according to the method of my present invention. This method briefly, comprises using the sheet steel, or other metal object which is to receive the coating of metal, as an anode in abath of strong sulphuric acid, this bath preferably being heated to-a temperature above normal, as between about 100 F and 130 F., preferably about 130 F. For the cathode I may use either another'sheet of steel or, and preferably, a sheet of lead. It is desirable that the sulphuric acid bath should be of a strength to more than about 86%. In particular, I find that sulphuric acid of about 92% strength,-or even anhy- 'drous,"or substantially anhydrous, as 06%, -'sulpln1r1c-ac1d' 15 very desirable for the treatment of the surface of a steel sheet or equivalent object preparatory to receiving. the coating of metal.

Y The polarization balance voltage of such a cell, operating at a temperature of about 130 -F., which voltage is equal to the back voltage.

resistance balance voltage generated by the cell when it reaches a condition of equilibrium, which will be-in about 15 seconds after it has been put into operation, is about 24 volts. I 'n'efer to use an overvoltage in excess of t epolarization balance voltage referred to, and amounting to a line voltage of about 42 volts. This results in an initial current density .of about 100 amperes per square foot where a temperature of about 130 F., -for example, has been employed. In about 15 seconds the cell, after a brief and initially slight increase in current den-' or about 17 amperes per square foot. After this condition of equilibrium has been attained, the treatment is continued for a tration of the acid in the bath, and amounting to an additional pepiod of treatment equal to about 180 seconds where an acid of "86% strength has been employed or to about seconds where an acid of about 92% Strength has been employed, and to about 15 seconds where an acid of about 96%- strength has been employed, making a total period of treatment of 195, and 30 seconds, respectively.

-Thesurface thus treated will be found to be free from grease, vegetable, animal and mineral oils, and to be adapted to render the subsequent coating remarkably sound, and adherent. Part of this desirable condition of the surface of the metal is, however,

comparatively temporary, especially in so far as the desirable property of adherence is concerned, although substantially permane'nt inso far as the desirable property of the imperviousness of the resulting coating is concerned. Although I do not wish to be limited to ,any particular theory'of the explanation of this phenomena, it is my present belief that the disappearance of the desirable property of adherence by exposure of the treated surface to the air, is due to the" probable fact that the adherence is a function of the oxidation characteristics of the surface to be coated, my treatment probably causing a reduction of the oxides on the surface of the metal to be coated, and thus permitting close adherence amounting even to analloy union between the base and the coating, so long as the surface of the base is in its original reduced condition. The impervious character of the coatingis,

on the other hand, probably the result of the changed character of the crystal surface of the iron or steel base, which surface is probably, by my treatment, deprived of its original surface-carbon content, and appears to be converted into ferrite. It is therefore preferred that the metal shouldbe at once coated by being placed in the deposition bath, preferably within about a minute after ithas been treated, as above set forth, and its surface-rinsedin water. The above-described characteristics of the surface of'the object to be coated may, however, be preserved, by keeping the metal under distilled water. or in' a 10% solution of sodium cy anide,

lVhere the'metal base, as sheet steel, has a large amount of-oxlde on 1t, it may be given a prehniinary pickling treatmentto remove the oxide. For example, I may use ed to the cleaning treatment described. length of time dependent upon the concen- I have discovered that my cleaning treatment successfully counteracts and overcomes the usual undesirable consequences of the ordinary pickling step, the ordinary pickling operation rendering the object subjected to the pickling step brittle and the subseent, notwithstanding the preliminary pickling treatment.

I'now provide'the steel or equivalent object treated as above set forth, with a dense, reguline, workable coating of zinc by any suitable method for electrodepositing zinc. Preferably I employ the following method of electrodepositing the zinc on the iron or steel object, since I have discovered that this method co-operates with remarkable efliciency' with my above-described method of treating the iron or steel object in the production of a coating which is impervious down to the smallest thicknesses and remarkably adherent. For this purpose I may employ a zinc bath containing, for each liter of electrolyte, three normal equivalents or approximately 242 grams ofzinc sulphate (ZnSO,), calculated in the dry state, of a mol. or 6.2 grams of boric acid (H BQ and of a mol. or 36 grams of dextrose (0 11 0 Th: solution is preferably worked cold, that is, at normal temperatures, with a current density approximately equal to 14 amperes per square foot of cathode surface. Any suitable zinc anode may be used. i 'The thickness of the coating applied as above set forth of course depends on the ultimate use to which the coated object is intended to be applied. Where the iron, steel or equivalent base has been treated in accordance with the method of my present invention, as aboveset forth, I have foundthatcoatings of z inc,'applied as herein set forth, or by any equivalent ethod for electrodepositing zinc, applying the zinc to the base, in thicknesses down to one-half of one ten-thousandth of or by or; rwise an inch. are impervious, while at the-same without in any way undesirably affecting, For example, I have its adherent qualities. applied to a sheet steel base havmga thickness of about 1/50 of an inch, a zinc coating of equal thickness, by the methods above set forth, and have found that in' such a case the coating metal could be separated from the coated -metalor base only; by chiseling the same apart, or by some slm'ilarly severe operation. Thereafter I found that I could not eifect a further separation of the coating from the base as the pull necessary to effect a further separation exceeded the strength ofthe coating metal,- in this case zinc.

The imperviousness of the coated metal may be demonstrated by any of the standard tests for porosity, which tests, when properly applied, will indicate that the coating, when made in accordance with the method of my present invention, is impervious down to remarkably small thicknesses. The workability of the coated sheet steel or other object made in accordance with the present invention, may readily be demonstrated by bending, punching and seaming the coated sheet. I have also found that sheets coated with zinc, in accordancewith the method described above, may readily and successfull be soldered by means of the usual sold'ers without the use of a flux. The coating being, in the example hero given, of zinc, and being impervious down to the very low limit of one ten-thousandth of an inch, not only gives an ornamental and highly attractive appearance to the metal base to which the same is applied, but also cfiiciently protects the same from rusting and from'the'action of the usual weathering agents. Furthermore, where the coating of zinceis applied by electrode deposition in accordance with the method above set forth, the coating will be found to possess an exceedingly fine grain, resulting in a fine, smooth coating, having unusual lustre and being unusually attractive in appearance.

Two of the phenomena which further show that my new cleaning treatment produces a change in the surface condition of the steel, and which are more noticeable in the case of steel which has been pickled, are: First, that the surface of the steel has an entirely diiferent appearance to the eye from any other steel,'treated or untreated, and especially from the same steel after pickling, being much whiter and more pearly and second, that in electrode depositing upon such treated steel, a condition which is particularly noticeable in the case of a thin'deposition, the deposited coatings of a clear, metallic-like lustre are formed upon the steel almost instantly, often within one I or two seconds, whereas in the case of steel,

not so treated, and particularly the case of forth for purposes ofillustration only.

What I claim is: 1. The method of preparing iron, steel or equivalentsurfaces to receive adherent and impervious metallic coatings, which comprises making the object to be coated the anode in a bath containing sulphuric acid of more than about eighty-six per cent strength wlnle maintaining the bath at a temperature above normal.

2, The method of preparing iron, steel or equivalent surfaces to receive adherent and IIIIPGI'VIOUS] metallic coatings, which comprises making the object to be coated the anodeina bath containing sulphuric acid of about ninety-two per cent strength while maintaining the bath at a temperature above normal.

3. The method of preparing iron, steel or equivalent surfaces to receive adherent and impervious -metallic coatings, which comprises making the object to be coated the anode in a bath containing sulphuric acid of more than about eighty-six per cent strength while maintaining the bath at a temperature pbove about one hundred degrees Fahrenieit.

4. The method of preparing iron, steel or equivalent surfaces to receive adherent and impervious metallic coatings, which com prises making the object to be coated the anode in a bath containing sulphuric acid of more. than about eighty-six per cent strength while maintaining the bath at a temperature of about one hundred andthirtydegrees Fahrenheit.

- 5. The method of preparing iron, steel or equivalent surfaces to receive adherent and impervious metallic coatings, which comprises making the object to be coated the anode in a bath containing sulphuric-acid of more than about eighty-six per cent strength, and impressing on the cell a'line voltage-substantially in excess of the polarization balance voltage of the cell. v,

6. The method of preparing iron, steel or equivalent surfaces to receiveadherent and impervious metallic coatings, which comprises making the object to be coated the anode in a bath containing sulphuric acidof more than about eighty-six per cent strength, and impressing on the cell a line voltage substantially in excess of aboutv twenty-four more than about eighty-six per centstrength,

and impressing on the cell a linevoltage of about forty-two volts.

8. The method of preparing iron, steel or equivalent surfaces to receive adherent and impervious metallic coatings, which comprises making the object to be coated the anode in a bath containing sulphuric acid of more than about eighty-sniper cent strength, impressing on the cell a line voltage substantially in excess of the polarization balance voltage of the cell, and maintaining said impressed-voltage for a'substantial time after the cell reaches a condition'of equilibrium.

9. The method of preparing iron, steel or equivalent surfaces to receive adherent and impervious metallic coatings, which comprlses making the object to be coated the anode in a bath containing sulphuric acid of more than about eighty-six per cent strength, impressing on the cell a line voltage substantially in excess of about twenty-four volts, and maintaining said impressed voltage for a substantial time after the cell' reaches a condition of equilibrium.

In testimony whereof, I have signed my name to this specification this 23rd day of July, 1920. i

CHARLES P. MADSEN'. 

